Cosmic microwave background (CMB) anisotropies probe the primordial density
field at the edge of the observable Universe. There is a limiting precision
(``cosmic variance'') with which anisotropies can determine the amplitude of
primordial mass fluctuations. This arises because the surface of last scatter
(SLS) probes only a finite two-dimensional slice of the Universe. Probing other
SLSs observed from different locations in the Universe would reduce the cosmic
variance. In particular, the polarization of CMB photons scattered by the
electron gas in a cluster of galaxies provides a measurement of the CMB
quadrupole moment seen by the cluster. Therefore, CMB polarization measurements
toward many clusters would probe the anisotropy on a variety of SLSs within the
observable Universe, and hence reduce the cosmic-variance uncertainty.Comment: 6 pages, RevTeX, with two postscript figure
